Section 28N. Notwithstanding the provisions of sections one to twenty-eight, inclusive, to the contrary, any correction or jail officer employed by county sheriffs’ offices who has performed services in said office for not less than twenty years shall, at his own request, be retired by said retirement board. Upon retirement under the provisions of this section a member shall receive a retirement allowance to become effective on the date of his retirement. Payments under such allowance shall be made as provided for in sections twelve and thirteen and the normal yearly amount thereof shall be equal to one-half of the annual average rate of his regular compensation during the twelve-month period of his creditable service immediately preceding the date his retirement allowance becomes effective; provided, however, that the total amount of the allowance shall be increased by one-twelfth of one percent for each full month of service in excess of twenty years service and prior to the last day of the month in which such member will attain the age of retirement. Any member retired under the provisions of this section who is a veteran as defined in section one shall receive an additional yearly retirement allowance of fifteen dollars for each year of creditable service or fraction thereof; provided, however, that the total amount of said additional retirement allowance shall not exceed three hundred dollars in any case.